A magnetic disk in a hard disk drive utilizes a magnetic thin film having a CoPtCr-oxide-based granular structure as a magnetic recording film that records information signals (for example, see Non-Patent Literature 1). In order to further improve the recording density of this magnetic thin film, there is a need to micronize magnetic crystal grains contained in the magnetic recording layer (magnetic thin film).
However, the progress of micronization of magnetic crystal grains has resulted in occurrence of the so-called thermal fluctuation phenomenon, in which the superparamagnetic phenomenon impairs the thermal stability of recorded signals and leads to loss of the recorded signals. This thermal fluctuation phenomenon has contributed a major obstacle to increasing the recording density of magnetic disks.
In order to surmount this obstacle, the magnetic energy of each magnetic crystal grain needs to be increased such that the magnetic energy exceeds the thermal energy. The magnetic energy of each magnetic crystal grain is obtained by v×Ku, which is a product of the volume v of the magnetic crystal grain and the magnetocrystalline anisotropy constant Ku. Therefore, the magnetocrystalline anisotropy constant Ku of the magnetic crystal grain needs to be increased in order to increase the magnetic energy (for example, see Non-Patent Literature 2).
Current perpendicular magnetic recording films have a granular structure composed of columnar CoPtCr crystal grains and crystal grain boundaries formed of an oxide that surrounds the columnar CoPtCr crystal grains. In order to increase the magnetocrystalline anisotropy constant Ku of the CoPtCr-oxide film having this granular structure, an attempt to perform sputtering with the substrate temperature elevated has been made. Sputtering with the substrate temperature elevated is intended to promote diffusion of metal atoms (Co atom, Pt atom, Cr atom) during film deposition and thus to obtain favorable magnetic crystal grains.
However, when the CoPtCr-oxide film is formed by performing sputtering with the substrate temperature elevated, the CoPtCr magnetic crystal grains grow not in a columnar shape but in spherical shape, which generates a phenomenon in which the magnetic crystal grains have a low magnetic anisotropy and the obtained CoPtCr-oxide film has a reduced magnetocrystalline anisotropy constant Ku (see Non-Patent Literature 3).
Therefore, the inventors of the present invention have attempted to improve the magnetocrystalline anisotropy constant Ku by performing annealing after forming a CoPtCr-oxide film (at room temperature) without raising the temperature of the substrate. However, annealing has increased the diameter of the lower portions (portions near the substrate) of columnar CoPtCr magnetic crystal grains and has caused magnetic coupling in the lower portions of the CoPtCr magnetic crystal grains. This has increased the interaction between the grains and has undesirably reduced the coercive force Hc.